For example, a developing device which develops an electrostatic latent image on a photoreceptor surface into a visible form using a two-component developer including carrier and toner is often used in dry-type copying machines. In such a developing device, the toner is consumed during a developing operation, while the carrier is not consumed and remains in the developing device. Consequently, the carrier deteriorates as the carrier and the toner in the developing device are frequently agitated. Then, a resin coat layer on the surface of the carrier is removed and the toner adheres to the surface thereof. As a result, the charging performance of the developer is gradually degraded.
In order to solve such a problem, for example, Japanese Publication for Examined Patent Application No. 21591/1990 discloses a device which restrains the degradation of the charging performance by continuously supplying a small amount of carrier to the developing device as well as toner that is consumed during a developing operation.
For example, as illustrated in FIG. 55, a developing device 108 includes a developer container 116 having therein a rotatable developing roller 117 formed by a magnet roller, and a rotatable agitating roller 118. Developer held in the developer container 116 is composed of carrier and toner. The carrier is formed by a magnetic substance and includes a resin coat layer for restraining the toner from adhering to the carrier surface. When the carrier and the toner are agitated by the agitating roller 118, the toner is charged by friction. The developing roller 117 transports the carrier by attracting the carrier with a magnetic force and forming a magnetic brush. The toner attracted to the carrier by Coulomb force is supplied to a photoreceptor 104 and attracted to the electrostatic latent image on the photoreceptor 104, thereby developing the image. The length of the magnetic brush is regulated by a doctor 119.
An opening for the supply of the developer is formed in a top wall 116a of the developer container 116. A developer supply unit 120 is placed above the opening so that it fits into the opening. The developer supply unit 120 is separated into two rooms, a toner storage 120a and a carrier storage 120b. The toner storage 120a stores toner, and the carrier storage 120b stores carrier only.
Disposed at the bottom of the storages 120a and 120b are a developer supply roller 121 and a carrier supply roller 122. As the developer supply roller 121 and the carrier supply roller 122 are rotated, the toner and the carrier in the toner storage 120a and the carrier storage 120b flow downward into the developer container 116 during the time in which the rollers 121 and 122 are driven. The rollers 121 and 122 are driven according to a detection signal of a toner-concentration detecting sensor 123 incorporated into the developer container 116.
The excess developer caused by the supply of the carrier is discharged from the developer container 116 through a discharge opening 116c formed in a predetermined location of a developer wall section 116b by an overflow mechanism, and collected in a collecting container 124. By successively suppling the toner and carrier and discharging the developer in a repeated manner, the deteriorated developer in the developer container 116 is replaced with newly supplied toner and carrier. With this structure, the amount of developer in the developer container 116 is maintained uniform, the charging performance of the developer is maintained, and degradation of copy quality is restrained.
However, with the structure of the developing device 108, since the toner and the carrier are separately supplied to the developer container 116 by the detection signal of the toner-concentration detecting sensor 123, the toner and the carrier have not been sufficiently mixed immediately after the supplying process. If copying is performed using developer in such an insufficient mixing state, copy quality is degraded. For example, when copying is performed immediately after the supply of carrier, some areas may have very low densities. On the other hand, when copying is performed immediately after the supply of toner, images may become foggy or some toner spots may appear on a produced copy, resulting in degraded copy quality.
Another developing device has a developer supply unit 130. As illustrated in FIG. 56, the developer supply unit 130 stores in a developer storage 130a a high-dense developer including toner and carrier in a predetermined ratio, and supplies the developer in the developer container by rotating a developer supply roller 131 located under the developer storage 130a. This developing device has a simplified structure compared to that of the developing device shown in FIG. 55 but enables simultaneous supply of toner and carrier to the developer container 116.
However, the above-mentioned developing device suffers from the following drawbacks. Specifically, although the developer to be supplied has been arranged to have a predetermined toner concentration, when the developer is introduced into the developer storage 130a, the carrier whose specific gravity is larger than that of toner is deposited on the bottom. As a result, the balance of toner and carrier in the developer becomes uneven, the toner concentration becomes lower in a lower part 131 (indicated by hatching in FIG. 56) than in an upper part 132.
If developer having such an uneven toner concentration is supplied to the developer container 116, the amount of carrier in the developer in the developer container may abruptly increases or the toner concentration in the developer therein may be unexpectedly increased. Such variations in the toner concentration in developer which is to be supplied to the developer container 116 prevent expected replacement of the deteriorated developer, thereby failing to maintain a desirable replacement ratio. Consequently, the charging performance of the developer in the developing device varies, resulting in degraded copy quality.